In order to maximize the amount of oil derived from an oil well a process known as hydraulic pressure stimulation or, more commonly, formation fracturing is often employed. In formation fracturing, fluid is pumped under high pressure down the wellbore through a steel pipe having small perforations in order to create or perpetuate cracks in the adjacent subterranean rock formation. The fluid employed must be able to withstand exceptionally high shear forces. Gelled liquids, and particularly gelled hydrocarbons, are well-suited for this application. The fracturing fluid has entrained therein a particulate material called a proppant (e.g., sand or other particulate matter). The proppant particles become wedged in the cracks of the formation to keep the cracks open once the external pressure is released, thereby enabling continued production stimulation of the well. It is ideal from the vantage points of time economy and cost-savings to have the gelling of the hydrocarbon take place more or less continuously on-site or "on the fly" as the components are brought together as they are pumped down the well bore. Accordingly, it is both desirable and adavatageous that gellation occur as quickly as possible.
The viscosity of the hydrocarbon gel is important for proppant transport. Poor gel viscosity can lead to a phenomenon known as "screening out", whereby the gel is not sufficiently capable of suspending the proppant. Large quantities of proppant material, upwards of 3 to 15 pounds of sand per gallon of pumping fluid, can settle out inside the well bore, as well as in the fracture. If the proppant has dropped, or screened out, part of the created fracture is effectively closed when the external pressure is released. When screening out occurs, the fracturing process must be interupted and the well bore cleaned out, costing both significant time and expense.
Rapid gellation of hydrocarbon liquids is also beneficial when tanks or vessels carrying such liquids are damaged during transport and cause highly hazardous and environmentally damaging spillage. A fast gelling additive composition that can be added to the leaking volume of hydrocarbon liquid would serve to prevent or, at least, reduce or contain the spillage and the resultant damage. A variety of other applications exist which require the rapid gellation of hydrocarbon liquids, and to which the present invention would apply.
Several means for gelling hydrocarbon liquids are disclosed in the prior art. U.S. Pat. No. 5,417,287 to Smith et al. is directed to a method for fracturing a subterranean formation which involves adding to a hydrocarbon liquid (a) an organic phosphate of the formula HPO.sub.4 RR' where R is an alkyl or alkaryl group having from 6 to 18 carbon atoms and R' is hydrogen or an aryl, alkaryl or alkyl group having from 1 to 18 carbon atoms; and (b) a ferric salt.
Smith et al 5,614,010 teaches gelling agents suitable for use in methods of fracturing formations, comprising ferric salts, certain phosphate esters, a low molecular weight amine such as triethanolamine or triethylamine, and an optional surfactant. Smith '010, however, does not achieve, e.g., the impressive hydrocarbon viscosities achievable by the present methods and compositions, and moreover Smith uses twice as much phosphate ester and ferric ion as required herein.
Smith et al U.S. Pat. No. 5,647,900 discloses gelling agents for hydrocarbon gels comprising combinations of certain orthophosphate esters and a composition comprising a source of ferric ions, a C.sub.2 -C.sub.12 amine, and a polycarboxylic acid or salt thereof. However, the gels formed in the Smith et al inventions demonstrate lower (Marsh funnel) viscosities than those achieved by the gels of the present invention. Moreover, the Smith et al gels are formed using twice as much phosphate ester and ferric ion (1% of each relative to the volume of liquid hydrocarbon to be gelled) as compared to the invention as demonstrated, e.g., in the Examples of the invention.
European Patent Application No. 551021A1 to McCabe et al. is directed to gelling a hydrocarbon liquid by adding thereto an at least partially neutralized alkyl orthophosphate acid ester, a C.sub.8 -C.sub.18 surface active amine and C.sub.2 -C.sub.4 monohydric alcohol. The surface active amine employed includes alkyl and alkanol amines having from about 8-18 carbon atoms, N-heterocyclic amines, alkyl substituted derivatives of such heterocyclics and mixtures thereof. Amines having more than one nitrogen group are preferred and imidazoline, such as that prepared from the reaction of a tall oil fatty acid with diethylenetriamine, is most preferred.
U.S. Pat. No. 4,316,810 to Burnham is directed to a fracturing composition which is an aluminum salt of an oxaalkyl phosphate in an oil base liquid. Surface active agents are not disclosed.
U.S. Pat. No. 4,153,649 to Griffin is directed to the reaction product of a hydroxy ether and a pentavalent phosphorus compound and an alcohol. The hydroxy ether has the formula ROR.sub.1 OH wherein R is a C.sub.1 to C.sub.6 alkyl group, R.sub.1 is a C.sub.2 or C.sub.3 alkylene group and the total carbon atoms of R.sub.1 and R range from 3 to about 8. The disclosed reaction product may be employed in the gelling of hydrocarbon liquids when used with a compound containing a multivalent metal cation.
U.S. Pat. No. 5,271,464 to McCabe is directed to a method of plugging or sealing a subterranean formation by introducing a rapidly gelling hydrocarbon thereto. To the hydrocarbon is added a first component which is an at least partially neutralized alkyl orthophosphate ester and a second component which is the reaction product of an aqueous source of aluminum or ferric ions and a C.sub.8 -C.sub.18 surface active amine in the presence of a water miscible organic solvent. The surface active amine is as defined above for European Patent Application No. 551021A1, also to McCabe. The water miscible organic solvent is generally a monohydric alcohol.
U.S. Pat. No. 3,494,949 to Monroe et al. is directed to an additive for improving the viscosity of motor oils which is generally an aluminum salt of an alkyl orthophosphate.
U.S. Pat. No. 2,983,678 to Pellegrini et al. is directed to an additive for lubricating oils which is generally a rare earth metal salt of a diester phosphate.
U.S. Pat. Nos. 4,877,894, 5,057,233, 5,110,485, and 5,202,035 to Huddleston are related to phosphate esters as hydrocarbon gelling agents. The gelling agent is generally formed by first reacting phosphorus pentoxide with triethyl phosphate, followed by reaction with a mixed alcohol that may have a substantial hexanol component. The gelling agent may also be in the form of an aluminum salt, by reaction of the phosphate ester with aluminum sulfate in the presence of solvent. None of the gelling agents or systems of the Huddleston patents appreciate the benefit of adding an amine, e.g., oxyalkylated amine, or an amine blend enhancer in the gelling of liquid hydrocarbons. The Huddleston teachings are also devoid of appreciation of the benefits obtained by using a ferric salt.
U.S. Pat. No. 5,190,675 and EP 225,661 to Gross (Dowell Schlumberger) employ metal phosphate diesters in the gelling of liquid hydrocarbons. The metal phosphate diester is prepared by reacting a phosphorus pentoxide with a triethyl phosphate, followed by reaction with an alcohol to form the diester. The metal salt is formed in the presence of the hydrocarbon to be gelled by the addition of a non-aqueous source of aluminum, e.g., aluminum isopropoxide. The gelling agents of Gross are devoid of the presence of an amine enhancer and a crosslinking ferric salt, and accordingly, Gross' methods do not achieve the level of beneficial gelling properties achievable by the present invention.
While a variety of systems are available for gelling hydrocarbon liquids for the application discussed above, there exists a clear need in the art for a means of improving the known systems to achieve decreased gelling times and improved viscosity.